The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head to hold a substrate during chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a polishing surface, e.g., a rotating polishing pad or moving polishing belt. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate.
The effectiveness of a CMP process may be measured by its polishing rate (removal rate), and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The removal rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
The removal rate is determined by many factors, including the downward force pressing the substrate against the polishing pad. However, a uniform removal rate across the substrate does not necessarily result even if the downward force is applied uniformly. For example, one problem in CMP is the xe2x80x9ccenter slow effect,xe2x80x9d which results in underpolishing of a central portion of the substrate. Such an effect is typically associated with the substrates having tungsten and aluminum layers. Another problem is the xe2x80x9ccenter fast effect,xe2x80x9d which results in overpolishing of a central portion of the substrate. Such an effect is typically associated with the substrates having copper layers.
In one aspect, the invention is directed to a carrier head with a housing, a flexible membrane coupled to the housing, and one or more coils positioned above the flexible membrane. The flexible membrane has a substrate receiving surface to receive a substrate, and has magnetically sensitive particles distributed therein. The coils o generate magnetic fields to exert magnetic forces on the particles.
Implementations of the invention may include one or more of the following features. The flexible membrane and housing may define a pressurizable chamber to press the substrate against the polishing pad. Three coils may be positioned concentrically about an axis of rotation of the carrier head. The current passing through each of the three coils can be independently controlled. The magnetically sensitive particles may include iron.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a carrier head, a polishing surface, and one or more coils. The carrier head has a housing and a flexible membrane with magnetically sensitive particles distributed therein. The membrane is coupled to the housing and has a substrate receiving surface to receive a substrate. The polishing surface contacts and polishes the substrate received on the substrate receiving surface. The coils are positioned on a side of the polishing surface opposite the substrate to generate magnetic fields to exert magnetic forces on the particles.
Implementations of the invention may include one or more of the following features. A platen may support the polishing surface and the coils. The plurality of coils may be positioned concentrically about an axis of rotation of the platen.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus that includes a carrier head, a polishing surface, and one or more coils. The carrier head includes a housing, a flexible membrane coupled to the housing and having a substrate receiving surface to receive a substrate. The flexible membrane has magnetically sensitive particles distributed therein. The polishing surface is positioned to contact and polish the substrate received on the substrate receiving surface. The coils generate magnetic fields to exert magnetic forces on the particles in the membrane and affect a pressure on the substrate.
Implementations of the invention may include one or more of the following features. The coils may be are positioned on a side of the polishing surface opposite the substrate. A platen may support the polishing surface and the coils. The coils may be positioned in the carrier head. A plurality of voltage sources may each be coupled to one of a plurality of coils to independently control voltages applied to each of the coils. Current may flow through the coils in opposite directions. The magnetically sensitive particles may include iron. The magnetically sensitive particles may be distributed non-uniformly in the membrane.
In another aspect, the invention is directed to a method of polishing a substrate in which a substrate held by a carrier head is brought into contact with a polishing surface, relative motion is created between the polishing surface and the substrate, and magnetic fields are generated to control the pressure being applied to different portions of the substrate.
Implementations of the invention may include one or more of the following features. The carrier head may a flexible membrane with magnetically sensitive particles dispersed therein, and the magnetic fields may create magnetic forces on the particles. The flexible membrane may define a chamber, and wherein the chamber may be pressurized to apply a load to the substrate. A portion of the load being applied to the substrate may be cancelled or supplemented with the force exerted on the particles. The coils may be located in the carrier head or on a side of the polishing surface opposite the substrate. Each of the coils may be coupled to an independent voltage source to independently control voltages being applied to each of the coils.
Implementations of the invention may potentially include zero or more of the following advantages. The pressure applied to the substrate can be controlled without requiring complex pneumatics. Non-uniform pressures can be applied to the substrate to compensate for non-uniform polishing rates. The invention provides an increased removal-rate profile control, e.g., to compensate for the center fast effect and the center slow effect.
Other features and advantages of the invention will be apparent from the following description, including the drawings and claims.